Actual fuel economy lower than one by DIC

[adallak]

After a 360 miles trip the resulting average fuel economy according to the DIC is 26.1 MPG. I did reset that data right before the trip. Unfortunately, the real fuel economy appeared to be 19.5 MPG...

Could a leaking injector (injectors) make such a difference? I have measured the resistance of all the injectors, and it was within specs. The FPR does not leak.

What do you guys think?

[Ranger]

Define "real" fuel economy.

[adallak]

Define "real" fuel economy.

The one based on the gallons pumped at the station and on the miles on the trip odometer.

[caddypete]

Thats a tough one to get actual gallons used. When you filled your tank the first time

you don't know exactly where the fuel level was thererfore when you refill the tank

it isn't at the exact same level. there could be a 1/2 gallon difference.

[adallak]

Thats a tough one to get actual gallons used. When you filled your tank the first time

you don't know exactly where the fuel level was thererfore when you refill the tank

it isn't at the exact same level. there could be a 1/2 gallon difference.

I agree. But here we are talking about four gallon difference per the trip...

[Cadillac Jim]

The way that the average fuel economy figure works is that it divides the fuel used according to a fuel flow sensor by the odometer reading, from the time that the average fuel economy was last set to the time it is read. You have to reset the average fuel economy when you fill up, then read it when you fill up again.

One cold start with a drive around town to get to the freeway can give your gas mileage a real hit. If you reset your average fuel economy when you get on the freeway, you will get a higher reading than your fillup to fillup average, so you need to reset the average gas mileage when you fill up. Also, taking the average over several fillups will reduce the effect of the fuel gauge error of your calculator average.

I've found my actual fuel economy to be slightly higher than that given by the average gas mileage reading, particularly in hotter weather. That's more normal because propane and butane bubbles in the fuel can cause the fuel rate sensor to read very slightly high.

[adallak]

The way that the average fuel economy figure works is that it divides the fuel used according to a fuel flow sensor by the odometer reading, from the time that the average fuel economy was last set to the time it is read. You have to reset the average fuel economy when you fill up, then read it when you fill up again.

One cold start with a drive around town to get to the freeway can give your gas mileage a real hit. If you reset your average fuel economy when you get on the freeway, you will get a higher reading than your fillup to fillup average, so you need to reset the average gas mileage when you fill up. Also, taking the average over several fillups will reduce the effect of the fuel gauge error of your calculator average.

I've found my actual fuel economy to be slightly higher than that given by the average gas mileage reading, particularly in hotter weather. That's more normal because propane and butane bubbles in the fuel can cause the fuel rate sensor to read very slightly high.

Jim,

If you read my first post carefully you'll see I did reset the average right before the trip. Actually I reset it as soon as I got the highway with my engine warmed up and at the same time I zeroed the trip odometer to exclude cold start effects (despite I do not see how a cold start would effect the accuracy of the fuel economy by the DIC compared to real one. At cold starts one does use more fuel per mile.

Secondly, more than 6 MPG difference cannot be attributed to the fuel gauge reading accuracy... I took all the possible measures to take into account actual fuel used by the car over 360 miles.

The original question was whether a leaking injector (injectors) could cause such a big difference? the resistances of the injectors are within specs.

Thank you in advance for any input. I emphasize once again the FPR is not leaking.

[BodybyFisher]

JimD once wrote this:

The PCM wins hands down in terms of accuracy and consistent repeatability.

To oversimplify it, all the PCM has to do is add up all the injector pulses, convert that to volume of fuel delivered, and divide that into the distance driven. Since the PCM is controlling the injectors, the PCM knows how many times the injectors are pulsed open (total engine revolutions), and how many microseconds the injector was held open (pulse width). If an injector flows X amount of fuel in Y microseconds at Z fuel pressure, the rest of the math is easy.

The big variable in the filling the tank method is --when is the tank full? When the pump shuts off? When your hands are too cold? When you are too wet to stand there and fiddle with the pump handle any longer?

Over the long term, the difference between PCM calculations and how much fuel you actually put in the tank might be small. But there is no doubt in my mind which method is more accurate and consistent. To take it to the extreme, I keep a spreadsheet on everything I do to my cars. For the first 152,000 miles, the difference between the DIC display and gas station pump is 5%.

People claiming fuel mileage could be anything. Different drivers in different situations and conditions.

And here is a very good thread to read through

http://caddyinfo.ipbhost.com/index.php?sho...;hl=pulse+width

[Ranger]

I have to agree. I think the DIC is probably much more accurate and is more likely the "real" fuel economy.

[Cadillac Jim]

There are two likely sources of error between the DIC/PCM average gas mileage and the calculator average: the amount of miles and the amount of fuel, both over the time and distance for which the average gas mileage is calculated.

Zeroing the odometer and average gas mileage just as you get on the freeway with the car warmed up will make your calculator average number reflect the distance from that point but your gas consumption will still be from the time you filled up. Since the fuel gauge can read full for a few gallons -- it's least accurate when reading full, this is likely the biggest source of error. I can see a four gallon error happening this way. You must reset the odometer and the average gas mileage in the filling station before you start the car to get a good comparison between the DIC/PCM gas mileages. This is my best guess as to the discrepancy in gas mileage that you are asking about. Other possible but smaller sources of error follow.

According to BodybyFisher's last post, the gas flow "sensor" is the total injector ON time. My previous post (and my older cars) used an in-line fuel rate sensor based on a rotating black-and-white ball, an IR LED, and an LED sensor that counted ball rotations, which is less accurate and susceptible to slightly high readings from bubbles. The Northstar, and probably the 4.9, use the total injector ON time; I don't know of an inline sensor in my Northstar.

If the FPR is keeping the pressure between the manifold vacuum and the fuel injector input relatively constant at the desired value as it is designed to do, then the only variable is the flow rate through the injectors, which is controlled in the manufacturing process. For the DIC/PCM reading to be a lot higher than the real gas mileage, this number would have to be a lot lower than the actual gasoline consumption, meaning that gas would be getting into the intake through a path other than the fuel injectors. I would say that since you have checked and your FPR isn't leaking, that isn't it. If your car idles smoothly in Park and Neutral when hot and you have no codes, I would venture that your injectors aren't leaking, at least not anywhere near enough to account for a 6 gallon difference over 360 miles. Any additional restriction in your fuel injectors due to clogging or high gasoline viscosity might cause your DIC/PCM gas mileage to read very slightly high but not drastically low. The most likely source of error here is waiting until you get on the freeway with the car warmed up to reset the DIC/PCM average gas mileage, omitting the period of worst fuel economy from a cold start and city driving from the gas station to a warmed-up engine on the freeway, while the gas gauge stays on full.

The distance is measured by the PCM by counting the ticks from the vehicle speed sensor (VSS). Accuracy of the VSS is very, very good, judging by my speed readings from my GPS as compared to my speedometer readings at any legal speed. Possible but unlikely sources of error are missed ticks due to bad connections or poor sensor signal to the PCM (which would cause noticeable problems with the speedometer needle wavering or quirky transmission shifting, codes being set, etc.) Variations in the tire size can cause errors with the DIC/PCM distance numbers but if you have OEM tire size that isn't likely. In any case, both the calculator and the DIC/PCM distances are taken from the trip odometer. So, any error between DIC/PCM and calculator numbers here must be from mis-reading the numbers -- or waiting until you get on the freeway before resetting it, which will reduce your calculator average gas mileage while the DIC/PCM measures fuel economy only under the best conditions of highway or freeway driving.

My money is on the DIC/PCM numbers. I would try resetting it before starting the engine when you fill up and see if the discrepancy doesn't go away.

[caddypete]

I agree the DIC is pretty dang accurate, but if you read Adallac's responses 4 gallons over 360 mile run is a big discrepency. 4 gallons is a large amount of error 1/2 gallon I can understand. I would believe fuel is being lost somewhere

[Cadillac Jim]

I come up with 4.67 gallons being the difference between 19.5 and 26.1 mpg over 360 miles. With a cold start and in-town driving mpg of 16 mpg, this comes to 75 miles, and the actual in-town gas mileage can easily be 18 mpg or more. So, I throw out two more sources of error: mis-reading the gas pump figures, or incorrect gas pump reading. A gas pump can read bad numbers if the underground tank is nearly empty, or if it has lots of butane and there are a lot of bubbles in the fuel.

Again, if it starts and idles well hot and has no codes, I don't think that there can be enough leakage in the FPR and injectors combined to make this kind of difference.

Speaking of butane in the gasoline, if the gas tank isn't pressurized the lighter hydrocarbons that are sometimes used to increase octane and decrease emissions, sometimes even including a little butane, will boil off. Around town the EVAP system may fill up and vent, but on the highway the EVAP system will purge regularly and your car will burn all the lighter hydrocarbons.

Does anyone know whether the PCM includes fuel from the EVAP purging in the gas flow figures? I would think that it does, or the instantaneous mpg reading would go through the roof when the EVAP system is purging. It's easy enough to get because is has the MAF data and that with a constant 14:1 fuel-air ratio during EVAP purging give the PCM a pretty good handle on the amount of fuel coming in through the throttle body from EVAP purging.

[adallak]

Okay I wanted to keep it short but looks like I have to go to greater detail. lol

1. I have arrived to the gas station in the morning with engine at operational temperature.

2. There was 8 gal in the tank according to the DIC.

3. I added 10 gal (pump reading) and the DIC reading came to FULL. It does not show actual gallons after 16 gal.

4. I reset the average and was on highway 395 in one minute.

5. I drove all the way at 75-79 MPH. Kept an eye at the data showing how much gas I still have in teh tank. I did notice numbers change after 19-20 miles despite the average according to the DIC was around 26... It wa srout 95 in CT which is quite flat and I kept constant speed, so fuel would not splash all over confusing the meter in the tank.

6. I added some gas on my way back so that it would not be more than 16 gal and I could compare the data in the car and one from the pump. The nambers matched.

7. After I have arrived to home (which is just a mile from the highway) I made all the calculations to make sure the real gas I used is much more than one according to the DIC. Yes, it was more than 4 gallons. I took 4 gallons just to eliminate errors associated with gas tank meter.

I do think 4 gallons discrepancy over 360 miles is a lot to ignore or explain with some errors. I think the culprit is one of injectors or a few of them. And one more time... gentlemen do you think injectors can leak THAT much? Thank you.

[BodybyFisher]

With this kind of gas consumption over and above what the PCM considers "optimum", I would think that a RICH code would be set, what does the OBDI spec do when the mixture is rich, anything?

[adallak]

With this kind of gas consumption over and above what the PCM considers "optimum", I would think that a RICH code would be set, what does the OBDI spec do when the mixture is rich, anything?

Good point, Mike. But i do not have any ECM codes stored. BTW, I need to check out the oxygen sensors.

[BodybyFisher]

I am about to eat, I have been trying to perfect a marinara sauce, , I will look in my 4.9 manual, I think there is a FUEL TRIM code... That is a lot of gas lost for something not to be wrong. Is the exhaust black? Have someone rev it while you watch the exhaust. How is it starting? You should have a few injectors passing too much fuel the way I see it

[Ranger]

The EVAP system only purges vapors so I seriously doubt that it is included in the calculations.

That is a pretty big error to be a leaky injector(s). I too would think that you'd have some driveability or idle problems, along with DTC's and sooty exhaust tips.

[BodybyFisher]

The EVAP system only purges vapors so I seriously doubt that it is included in the calculations.

That is a pretty big error to be a leaky injector(s). I too would think that you'd have some driveability or idle problems, along with DTC's and sooty exhaust tips.

This much fuel should set off an O2 sensor RICH code because the O2 sensor can no longer compensate. I would think you would get one of these OBDl codes with that kind of consumption

P043 (E043) ......................................... Left Oxygen Sensor Signal Rich

P045 (E045) ........................................ Right Oxygen Sensor Signal Rich

P046 (E046) .................................. Right to Left Bank Fueling Difference

P080 (E080) ....................................................... Fuel System Rich

By the way, the chunky marinara sauce was so so, watery and not tasty, I marked the details and results down in my computer, and will try again with a different tomato and less water.... I'LL GET IT YET The Pinot Noir was good however,

[Cadillac Jim]

Several of us including myself agree that adillac's car doesn't have significant leakage in the injectors or FPR if there are no driveability problems or codes.

I would not trust the DIC reading for the amount of gas in the tank to be accurate, even for 4 gallons. The fact that adillac put in 8 gallons when the PCM said that 4 gallons was present is evidence that the notorious sloppy readings of gas gauges in all cars applies to Cadillacs as well; I trust only Full, Empty, and below 1/4 tank. When my car was new I ran out of gas with never a "Low Fuel" warning while cruising on an Interstate; it read 1/8 tank when it ran out (Sticky gas sensor? I guess so!). What I would trust for getting accuracy better than one gallon is the DIC or gas gauge reading the same place, below 1/4 tank, at successive fillups.

I believe that the instantaneous gas mileage works when the EVAP system is purging or we would have heard about it here on caddyinfo.com forums long ago, because the instantaneous gas mileage reading would go through the roof during purge if it didn't. Since the EVAP canister can hold the equivalent of one or two gallons of gas, then there is another source of discrepancy, particularly if a car is driven in the city for a week or more to give the EVAP canister time to fill up without an opportunity to purge.

What I see in all the posts is a common thread of what was voiced in two early posts in this thread, both by Ranger:

Define "real" fuel economy.

I have to agree. I think the DIC is probably much more accurate and is more likely the "real" fuel economy.

Thus what we are all doing is looking for the error in the fillup-to-fillup calculator average, and adillac is concerned that it might be injectors leaking. Many of us agree that the injectors are almost certainly OK. What's left is identification of the errors in the gas used, and we have some:

• The DIC reading of remaining fuel may not have read the same between fillups, and the reading is demonstrably an approximation; the car doesn't hold 18 gallons, I don't believe.
• The EVAP canister may have held the equivalent of a gallon or more, which would have been accounted for by the DIC/PCM average fuel economy but not the calculator average. We know that the instantaneous fuel economy works during EVAP purge so the average fuel economy must work too, using the fuel in the EVAP canister.

So what we have is disbelief that over 4.5 gallons of error in this process could occur. I think that the whole situation can be summed up as

• There is likely nothing wroing with adillac's car.
• We can't account for over 4.5 gallons of discrepancy over a 360 mile trip.
• The DIC reading of fuel available, EVAP canister estimated capacity of the equivalent of two gallons, and whatever else we can come up with add up to about 4 gallons, max.
• This situation calls for a re-test.
• We have looked at the situation enough to recommend these conditions for a re-test:

Drive on the highway for at least an hour, perhpas one tankful, to make sure that the EVAP is purged. Then, fill up and note the gas reading from the PCM and gas gauge, reset or record the trip odometer reading, and reset the average fuel economy. Record all gas added until you fill up when the PCM and gas gauge read within a gallon of the reading at the start of the test, then use these figures and compare them to the PCM average fuel economy reading.

[BodybyFisher]

Wait a minute, am I to understand that the DIC was used to determine how much fuel was in the tank to do an estimate of how much fuel was consumed?

The proper way to do this test is to fill it up slowly till the pump clicks and stop. Record the mileage or reset the TRIP MILEAGE to 000. Take the trip or drive around, fill it up (at the same pump if possible ) slowly the same way, stopping when the pump clicks once, READ THE GALLONS OFF THE GAS STATION PUMP, and divide that into the miles that you drove or the miles on your TRIP MILEAGE if you set it. What method was used, maybe I didnt pick up the method, I have to look back..

[Cadillac Jim]

BodybyFisher -- calibrating the PCM reading for a particular gas level sensor position will work for differences in fuel level that are measured at or near the level where you did the calibration, but the fuel level sensor isn't linear. Calibrating for near 1/4 tank won't tell you how many gallons are really there when the gauge reads 1/4 tank, just that when the PCM reads 4 and 5.25 gallons that there is really more like 1 gallon difference. The only way to get a good calibration is to put in 1 gallon at time, starting at empty, and read the PCM for every gallon (after shaking the car to get past any sticking in the tank sensor) and get the curve.

This is why the Range reading, the first one to sequence through on the DIC when you press the Info button on the analog cluster, is very approximate and simply changes to "Range Low" when the PCM sees less than about 1.5 gallons.

[BodybyFisher]

My fuel gage is terribly inaccurate and I would never attempt to use it to determine fuel used by looking at what the gage says I have left. There have been days mine gage says 6 gallons and the next minute 3, then back to 6 or 5. I need to change mine.

[bobbnsueb@sbcglobal.net]

Hi,

My 91 STS had the same large discrepancy when we first bought it.

After a lot of discussion with the GURU and others, I measured the resistance of each injector and found a few to be very low.

I ran the cylinder balance and found one injector not shutting off as it should.

New injectors fixed the problem.

Bob B

[adallak]

My fuel gage is terribly inaccurate and I would never attempt to use it to determine fuel used by looking at what the gage says I have left. There have been days mine gage says 6 gallons and the next minute 3, then back to 6 or 5. I need to change mine.

Mike, I think your gauge is not any fifferent from others. Mine does the same fluctuations.

[adallak]

Hi,

My 91 STS had the same large discrepancy when we first bought it.

After a lot of discussion with the GURU and others, I measured the resistance of each injector and found a few to be very low.

I ran the cylinder balance and found one injector not shutting off as it should.

New injectors fixed the problem.

Bob B

Bob B,

This is the reply I have been waiting for! I did measure the resistance a few months ago. Looks like I need to do that again.